In mobile communications systems, user terminal (UT) position determination based on signal propagation delay and Doppler shift is known. Typically, the propagation delay and Doppler shift are derived from a radio-frequency (RF) carrier transmitted between the UT and a moving transceiver, for example, a transceiver included on a moving airplane or satellite. The Doppler shift is a well known physical phenomenon and represents the observed change in frequency of the propagated RF wave that occurs due to the relative motion between the UT and the transceiver. The measured signal propagation delay is the amount of time required for an electromagnetic signal to travel between the UT and the moving transceiver. From this delay, it is easy to calculate the distance separating the UT and transceiver by multiplying the delay by the speed at which the electromagnetic signal travels, which is generally at or near the speed of light.
One approach to determining UT position is to represent the signal propagation delay and Doppler shift as a system of equations that are functions of the UT position. For example, the propagation delay can be a function f of the UT position, while the Doppler shift can be a function g of the same. EQU propagation delay=f (UT position) EQU Doppler shift=g (UT position)
To determine the UT location using this approach, the system of equations is solved for the UT position. This is usually an iterative procedure, such as the Newton-Raphson Method, that numerically searches for an approximation of the UT position. Such an approach is computationally intensive, requiring substantial computer resources, such as processor bandwidth, memory, and the like. Consequently, this approach is impracticable in systems where computational resources and response times are limited, such as mobile communication systems. For instance, in satellite mobile communication systems, it may be necessary to perform 100-200 UT position determinations per second during peak usage. Another drawback to this approach is that its search uses two pieces of information, namely, the delay and Doppler, which yields two possible UT positions, and thus two possible solutions to the system of equations. Such situations are generally fatal to numerical search techniques, as they often cannot converge or converge to the wrong solution.
Another conventional approach to determining the UT position is to rely on the Global Position System (GPS). However, GPS receivers are expensive. Requiring each UT in a communication system to include a GPS receiver would dramatically increase costs. In addition, on cold-start power up, it often takes a GPS receiver several minutes to acquire its position. This lengthy acquisition time is impracticable in many applications. Moreover, in such a system, each UT position determination may need to be performed at the beginning of a phone call without noticeable delay to the user.